Articulated structural support linkage

ABSTRACT

Offshore apparatus is disclosed which includes a base, a tower pivotally connected to the base, at least one support member attached to the tower and adapted to slidably support a conductor pipe, and at least one conductor pipe adjacent and substantially parallel to the longitudinal axis of the tower and laterally supported by at least one support member. An articulatable means connects the apparatus to a span of the conductor pipe situated in close proximity to the pivotal connection. This means provides sufficient lateral support to the pipe span to prevent buckling and at the same time controls the configuration of the pipe span as the tower sways to prevent bending stress within the pipe span from attaining a level that will cause the pipe to fail.

United States Patent [451 June 20, 1972 Lloyd [54] ARTHCULATEDSTRUCTURAL SUPPORT LINKAGE [72] Inventor: James R. Lloyd, Houston, Tex.

[73] Assignee: Esso Production Research Company [22] Filed: Sept. 2,1970 [2]] Appl. No.: 69,066

[52] US. Cl ..6l/46.5, 175/9 [51] .....E02d 21/00, E2lb7/12 [58] FieldoiSearch ..61/46, 46.5; 175/5, 7,8,9; 166/5, .6

[56] References Cited UNITED STATES PATENTS 3,524,323 8/1970 Miller..6l/46.5 3,472,032 10/1969 Howard ..6l/46 Primary Examiner-JacobShapiro Attorney-James A. Reilly, John B. Davidson, Lewis H. Eatherton,James E. Gilchrist, Robert L. Graham and James E. Reed ABSTRACT Offshoreapparatus is disclosed which includes a base, a tower pivotallyconnected to the base, at least one support member attached to the towerand adapted to slidably support a conductor pipe, and at least oneconductor pipe adjacent and substantially parallel to the longitudinalaxis of the tower and laterally supported by at least one supportmember. An articulatable means connects the apparatus to a span of theconductor pipe situated in close proximity to the pivotal connection.This means provides sufficient lateral support to the pipe span toprevent buckling and at the same time controls the configuration of thepipe span as the tower sways to prevent bending stress within the pipespan from attaining a level that will cause the pipe to fail.

4 Claims, 2 Drawing Figures PATENTEDJM 20 1972 3.6 70 51 5 sum 1 or 2IIIHIIIIIIIE' INVENTOR. JAMES R. LLOYD ATTORNEY P'A'TENTEBuunzo I8723.670.515

sum 2 or 2 INVENTOR. JAMES R. LLOYD ATTORNEY BACKGROUND OF THEINVENTION 1. Field of the Invention This invention is generally directedto an improved tower for use in conjunction with the production of crudeoil and natural gas offshore and is particularly concerned with asupport linkage which prevents conductor pipes associated with such atower from buckling under axial loading while reducing bending stressesby forcing them to flex with a large radius of curvature.

2. Description of the Prior Art As efforts directed towards drilling forand production of crude oil and natural gas are extended towards deeperwaters, depths will ultimately be reached which will make it impracticalto use the rigid, bottom-founded platforms presently employed to supportsuch operations. One deep water alternative to these rigid platforms isa buoyantly supported tower. These structures generally include a basefixed to the submerged bottom, an elongated tower provided with one ormore buoyancy chambers near its upper end, and a pivot that connects thetower to the base and permits the tower to sway in response to wind,waves and ocean currents.

Wells drilled from such towers would normally be completed with thewellheads either on the ocean bottom or above the water surface. Priorart buoyantly supported towers nonnally have wellheads that are mountedon the base of the structure or on the ocean floor. In deep water,placement of underwater wellheads near the base of buoyant towers makesaccess difficult and will require extension of diver-operating depths aswell as substantial advances in undersea well maintenance technology.Surface well completions would therefore be preferable because of theaccessibility of the wells for maintenance, workovers and the like. Butsurface completions have the disadvantage that each well requires anindependent conductor pipe that extends from the water surface to theocean floor. These conductor pipes must bend as the tower sways and theythereby develop high stresses which may lead to their failure. Thus,surface completions, though preferable, are frequently beset withexcessive bending stresses in the conductor pipes, and thus leavesomething to be desired for deep water operations.

SUMMARY OF THE INVENTION The apparatus of the invention reducesconductor pipe stresses and thus will generally alleviate the problemswith surface well completions outlined above. The present invention isdirected to offshore apparatus comprising a base, a tower pivotallyconnected to the base, at least one support member attached to the towerand adapted to slidably support a conductor pipe, at least one conductorpipe adjacent and substantially parallel to the longitudinal axis of thetower and laterally supported by at least one support member, and meansarticulatably connecting the apparatus to a span of conductor pipesituated in close proximity to the pivotal connection. The articulatingmeans provides sufficient lateral support to the pipe span to preventbuckling and controls the configuration of the pipe span as the towersways to prevent bending stress within the pipe span from attaining alevel that will cause the pipe to fail.

The permissible level of stress within a conductor pipe as it bendsthrough a given angle of tower sway is limited by two factors: l) acritical radius of curvature which gives rise to an excessive bendingstress, and (2) a critical unsupported length of conductor pipe whichbuckles under axial compressive loads. To increase the radius ofcurvature of the conductor pipe as it conforms to the angle of towersway requires lengthening the unsupported span of conductor pipe;however, this in turn increases the likelihood of the conductor pipebuckling. The apparatus of the invention overcomes the problems existingin prior art towers by providing an articulatable lateral conductor pipesupport that both prevents buckling and moves as the tower sways,forcing the conductor pipe to bend with a large radius of curvature. Itwill therefore be apparent that a buoyantly supported tower constructedin accordance with the invention will have significant advantages oversimilar structures disclosed in the prior art.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic elevation of abuoyantly supported tower having two conductor pipes that extend fromthe water surface to the ocean bottom and that are supported by thelinkage of the invention.

FIG. 2 is an enlarged perspective view of the lower part of a buoyantlysupported tower equipped with a conductor pipe support linkageconstructed in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 schematically depicts abuoyantly supported tower for use in conjunction with the ofishoreproduction of crude oil and natural gas. A base 11 is shown anchored bymeans of pilings 12 to submerged bottom 13. An elongated tower 15 havinga buoyancy chamber 17 near its upper end is connected to the base by apivot assembly 19. A platform 21 atop the tower and above the watersurface supports a drilling rig 23. Conductor pipes 25 are situatedadjacent the side of the tower and substantially parallel to itslongitudinal axis and are shown extending from the platform to thesubmerged bottom. A number of conductor pipe supports 27 are fixed tothe tower structure and provide lateral support for the conductor pipesto prevent them from buckling as well as restrain them from lateralmovement. These supports permit the conductors to slide freely throughthem so that tower sway will not impart excessive axial loads to theconductors. The conductor pipe support linkage of. the invention,designated by numeral 29 and shown attached to the tower near its lowerend, supports the span of conductor pipe between the lowermostfixedconductor pipe support and the ocean floor. The most severe bendingstresses will occur in this span of conductor pipe because this spanexperiences the sharpest radius of-curvature. In addition, the axialcompressive load that is effective in causing buckling is at a maximumat the bottom of the tower because of the weight of the conductor pipe,internal fluids and other pipe strings suspended within the conductorpipe.

The maximum permissible unsupported length of conductor pipe iscontrolled by the tendency of the conductor pipe to buckle. Assuming thelowermost span is clamped at the bottom where it emerges from the oceanfloor, and is pinned, i.e., subject to no moments, at the first fixedsupport, the conductor pipe will be in stable equilibrium provided theaxial load is lower than P as defined by the following relation:

P the maximum permissible axial compressive load for a condition ofstable equilibrium E modulus of elasticity of pipe material I moment ofinertia of the effective conductor pipe cross section L unsupportedlength of the conductor pipe c 20.2, a constant consistent with theassumed end conditions Inspection of the above relation will readilyindicate that decreasing the unsupported length by a factor of 2 willincrease the axial load that can be withstood without buckling by afactor of 4. This could be accomplished by slidably supporting thelowermost span at its midpoint with a support member that is fixed tothe tower, such as those depicted by numeral 27 in FIG. 1. However,shortening the unsupported span in this fashion is not desirable sincethe fixed support would force the pipe to bend more sharply for anygiven angle of tower sway, thereby reducing the radius of curvature ofthe pipe and substantially increasing the bending stress in the pipe.

Bending stress within the conductor pipe resulting from deflectioncaused by tower sway is governed by the following approximate relation:

where:

a-= bending stress in the pipe E modulus of elasticity of pipe materialL length of unsupported section bending angle UK (radians) R radius ofcurvature D pipe diameter It will be noted from the above that cuttingthe length of the unsupported section in half will have the effect ofdoubling the bending stress in the conductor pipe for a given angle oftower sway. Thus, while improving the resistance of the conductor pipeto buckling, the additional fixed lateral support would substantiallyincrease the bending stress within the pipe.

The conductor pipe support linkage depicted by numeral 29 in FIG. 1reduces the buckling tendency of the conductor pipes withoutsubstantially increasing the level of bending stress. The linkageaccomplishes this by slidably supporting the conductor pipes and, at thesame time, forcing them to bend with a large radius of curvature. Thesupports shown are each connected to the tower structure by a pair ofarticulated rigid members. One member is connected to the tower and theother to the tower base so that as the tower sways towards theconductor. pipe, the hinged support linkage thus formed pushes theconductor away from the tower. As the tower flexes in the oppositedirection, the hinged support linkage draws the conductor towards thetower. The articulated members that form the hingelike conductor pipesupport linkage are sized and positioned to maintain the unsupportedspan of each conductor pipe in a configuration that closely approaches acircular arc, regardless of the angle of tower sway. Preferably thelinkage is attached at a point about midway between the ocean floor andthe lowermost fixed conductor pipe support and is configured to forcethe unsupported pipe span between these two points to bend in aconfiguration approaching that of an are having a constant radius ofcurvature over any segment between the ends of the span. This radius ofcurvature should be smaller than that of any other segment along theconductor pipe. It will be apparent that when the tower sways, thehinged support linkage, if employed to slidably support the lowermostunsupported span near its center, will reduce the unsupported length ofthe span by a factor of 2 without substantially reducing the radius ofcurvature of the conductors during tower sway. Thus, for any given angleof sway, the axial load that can be tolerated without the conductorpipes buckling can be increased by a factor of approximately 4 withoutsubstantially increasing the bending stress. Conversely, for a givenangle of tower sway and for a given axial compressive load in theconductor pipe, the bending stress can be cut in half by using thissupport linkage.

It will be obvious that any number of these linkages could be employed,each further subdividing the unsupported span into shorter and shorterlengths without increasing the bending stress. It will also be obviousthat the incremental benefit to be derived from each additional linkagediminishes as the number of linkages increases.

FIG. 2 is an enlarged perspective view of a conductor pipe supportlinkage constructed in accordance with the invention. Identical numeralshave been used in FIG. 2 to designate elements of structurecorresponding to those shown in FIG. 1. Thus, a base section 11 is shownconnected to the lower portion of a buoyantly supported tower 15 bymeans of a pivot assembly 19. A number of conductor pipes extendupwardly from the base and are situated adjacent and substantiallyparallel to the longitudinal axis of the buoyantly supported tower. Thelast two fixed conductor pipe supports 27 are shown on the lower portionof the tower. These devices serve to slidably support the conductor pipeto prevent it from buckling and to restrain it from moving laterally.The conductor pipe will therefore conform to the configuration of thetower as it sways.

The conductor pipe support linkage of the invention is again generallydepicted by numeral 29. The linkage shown is designed to support sixconductor pipes; however, a larger (or smaller) number of conductorpipes could be handled in a similar fashion. The individual slidablesupports are comprised of sleeves 31, which pass through a polygonalframe 33 and may have a series of comer braces (not shown). Otherconfigurations of slidable supports and of the structural frameworkcould, of course, also be employed. The support structure is shownconnected to the tower by six struts or similar rigid members 37. Theserigid members are connected to the frame and to the tower structure byarticulatable joints 39, e.g., ball or universal joints. The articulatedrigid members are spaced around the hexagonal frame at 60 intervals andare connected alternatively to the tower section and the base section.Thus, three articulated rigid members connect the frame to the tower andthree articulated rigid members connect the frame to the base. Thearticulated rigid members are shown in FIG. 2 to be spaced evenly aboutthe tower axis. It will be apparent, however, that other configurationsof these articulated rigid members could be used, provided they supportthe frame in such a way as to provide lateral support for the conductorpipes and to force the pipes to bend in substantially circular arcs asthe tower sways.

What is claimed is:

l. Offshore apparatus comprising a tower structure including a baseaflixed to a marine bottom and a tower pivotally connected to said base;a conductor pipe anchored at least adjacent said bottom, situatedadjacent and substantially parallel to said tower, and supportedlaterally and slidably by at least one conductor pipe support attachedto said tower near the lower end thereof; and a means articulatelyconnecting said tower structure to a point proximate the midpoint of aspan of conductor pipe defined between said anchored point and saidlower conductor pipe support, said means providing sufficient lateralsupport to said pipe span to prevent buckling and controlling theconfiguration of said pipe span as the tower sways to prevent bendingstress within said pipe span from attaining a level that will cause thepipe to fall.

2. Apparatus as defined by claim 1 in which said articulatable meansforces said pipe span to bend in a configuration approaching that of anare having a constant radius of curvature over any segment between theends of said span, said radius of curvature being smaller than that ofany other segment along said conductor pipe.

3. Ofishore apparatus comprising a tower structure including a baseaffixed to a marine bottom and a tower pivotally connected to said base;at least one conductor pipe support attached to the tower and adapted toslidably support a conductor pipe, at least one conductor pipe adjacentand substan tially parallel to the longitudinal axis of the tower andlaterally supported by at least one said conductor pipe support, and ameans articulately connecting said tower structure to a span ofconductor pipe situated in close proximity to said pivotal connection,said means providing sufficient lateral support to said pipe span toprevent buckling and controlling the configuration of said pipe span asthe tower sways to prevent bending stress within the pipe span fromattaining a level that will cause the pipe to fail, said meansincluding,

a. a support frame situated in proximity to said pivotal connection andhaving at least one conductor pipe support member attached thereto;

b. a first plurality of rigid members spaced about said frame, each saidmember having one end articulately connected to said frame and theopposite end articulately connected to said base; and

c. a second plurality of rigid members spaced about said frame, eachsaid member having one end articulately connected to said frame and theopposite end articulately connected to said tower.

4. In an offshore structure of the type which includes a base anchoredto a submerged bottom, a first vertical member pivotally connected tothe base, a second vertical member affixed to and extending upwardlyfrom said bottom that is both adjacent and substantially parallel tosaid first vertical member and is laterally supported by at least onesupport member attached to said first vertical member, theimprovethereto;

ment comprising an articulatable means adapted to connect a b. a firstplurality of rigid members spaced about said frame, span of said secondvertical member proximate said pivotal each said rigid member having oneend articulately eonconnection to said structure, said means providingsufficient nected to said frame and the other articulately connectedlateral support to prevent said span from buckling and forcing 5 to saidbase; and

said span to bend with a radius of curvature such that bending a Secondplurality of rigid membm's spaced abom said stress within the span willnot attain a level that will cause said frame, each Said member havingone end to said frame and the other articulately connected to saidsecond vertical member to fail when said first member pivots,

first vertical member.

and said means including,

a. a frame having at least one support sleeve attached l0

1. Offshore apparatus comprising a tower structure including a baseaffixed to a marine bottom and a tower pivotally connected to said base;a conductor pipe anchored at least adjacent said bottom, situatedadjacent and substantially parallel to said tower, and supportedlaterally and slidably by at least one conductor pipe support attachedto said tower near the lower end thereof; and a means articulatelyconnecting said tower structure to a point proximate the midpoint of aspan of conductor pipe defined between said anchored point and saidlower conductor pipe support, said means providing sufficient lateralsupport to said pipe span to prevent buckling and controlling theconfiguration of said pipe span as the tower sways to prevent bendingstress within said pipe span from attaining a level that will cause thepipe to fail.
 2. Apparatus as defined by claim 1 in which saidarticulatable means forces said pipe span to bend in a configurationapproaching that of an arc having a constant radius of curvature overany segment between the ends of said span, said radius of curvaturebeing smaller than that of any other segment along said conductor pipE.3. Offshore apparatus comprising a tower structure including a baseaffixed to a marine bottom and a tower pivotally connected to said base;at least one conductor pipe support attached to the tower and adapted toslidably support a conductor pipe, at least one conductor pipe adjacentand substantially parallel to the longitudinal axis of the tower andlaterally supported by at least one said conductor pipe support, and ameans articulately connecting said tower structure to a span ofconductor pipe situated in close proximity to said pivotal connection,said means providing sufficient lateral support to said pipe span toprevent buckling and controlling the configuration of said pipe span asthe tower sways to prevent bending stress within the pipe span fromattaining a level that will cause the pipe to fail, said meansincluding, a. a support frame situated in proximity to said pivotalconnection and having at least one conductor pipe support memberattached thereto; b. a first plurality of rigid members spaced aboutsaid frame, each said member having one end articulately connected tosaid frame and the opposite end articulately connected to said base; andc. a second plurality of rigid members spaced about said frame, eachsaid member having one end articulately connected to said frame and theopposite end articulately connected to said tower.
 4. In an offshorestructure of the type which includes a base anchored to a submergedbottom, a first vertical member pivotally connected to the base, asecond vertical member affixed to and extending upwardly from saidbottom that is both adjacent and substantially parallel to said firstvertical member and is laterally supported by at least one supportmember attached to said first vertical member, the improvementcomprising an articulatable means adapted to connect a span of saidsecond vertical member proximate said pivotal connection to saidstructure, said means providing sufficient lateral support to preventsaid span from buckling and forcing said span to bend with a radius ofcurvature such that bending stress within the span will not attain alevel that will cause said second vertical member to fail when saidfirst member pivots, and said means including, a. a frame having atleast one support sleeve attached thereto; b. a first plurality of rigidmembers spaced about said frame, each said rigid member having one endarticulately connected to said frame and the other articulatelyconnected to said base; and c. a second plurality of rigid membersspaced about said frame, each said member having one end connected tosaid frame and the other articulately connected to said first verticalmember.